Fathometer, underwater device used on ships to measure the depth of water. As the speed of sound in water is known, the fathometer works by sending a sound down through the water, to be returned by echo. The fathometer dispatches sound through a submarine oscillator and receives it through a hydrophone echo receiver. First manufactured in 1927, the fathometer is the trade name for a refined sonic depth finder. The first sonic depth finder was developed in 1919 by the U.S. Navy.
See also: Sonar.